This invention relates to a strand and more particularly to a multiple-wire strand formed of high-tensile steel suitable for use in posttensioning prestressed concrete members or structures.
Although concrete has been utilized as a structural material since ancient time, it has been only recently (in the past 100 years) that concrete has been used as a primary building material. This advance was possible by the use of reinforcing iron bars placed in the lower parts of wooden forms which provided the required tension for girders, beams, and flooring after the concrete had been poured and hardened and the forms removed. This enabled the design and utilization of concrete members for use under both tension and compression conditions and led to the subsequent stressing of the concrete itself (prestressed concrete). Prestressed concrete is the name applied to concrete products that have been compressed by either pretensioning or posttensioning of high tensile steel wires, rods or strands that remain permanently imbedded in the concrete to couple the properties of tension, shear and torsion to the compression property of concrete.
There are two techniques used for prestressing concrete: Pretensioning and posttensioning. Pretensioning is usually restricted to high volume products that can readily be transported from point of manufacture to erection site. The forms used are equipped with high-strength bulkheads through which the uncovered, bare, and clean steel members are threaded and stressed. After stressing, the concrete is poured. High-early concrete is formulated for overnight curing (with heat if necessary) to a strength (usually 4000 P.S.I.) to grip and withstand the pressure of the steel when released from the bulkheads without crushing said concrete.
Posttensioning, or tensioning the steel members after pouring and hardening of concrete, can be produced in forms as described above, as well as in temporary forms at the construction site, by placing coated high tensile steel tendons in said forms in desired position after which the concrete is poured and given time to set up and cure to the point that it will withstand the stress that develops when the high-tensile tendons are stretched tight. This process requires that one tendon end be held securely while pulling the opposite end with a jacking device until the steel is elongated within its elastic limit (about 30,000 to 33,000 pounds on a 1/2" 7-wire strand of high carbon, high-tensile steel).
Presently a tendon in wide use is that described in U.S. Pat. No. 3,646,748 in which a high-tensile strand is encased in petroleum based grease or other lubricating corrosion inhibitors to protect the strand from corrosion as well as from abrasion by the encompasing concrete during tensionsioning of said strand. The thus grease coated strand is then provided with a tight fitting plastic jacket. These tendons are produced by passing the strand through a grease filled container just ahead of the extruder. The spiral convolutions of the outer wires of the strand scrape grease off the inner wall of the hole produced in the grease by the lineal passage of the strand. Because this application of grease is performed at the extrusion rate of up to 300 feet per minute, the actual time span of the strand in a grease container of about two feet long is less than one-half second, which results in the grease filling only the outer portions of the strand interstices and no grease in the inner voids. Because a tight jacket is extruded over the grease encased strand, a slight positive pressure is immediately exerted on the grease which then starts to migrate to the inner voids very slowly. The vibrations of shipping, the warmth of sun exposure, and the flexing of the strand during make-ready activity or otherwise during the time span prior to use will move the grease into the inner voids thus reducing the intended protection of the strands by enabling the jacket to enter the convolutions of the spiraled outer strand wires resulting in a more generally hexagonal-shaped tendon and thus an appreciable amount of additional friction during tensioning of the strand.
Also since these prior art tendons are encased in grease prior to plastic jacketing, they cannot be used in those instances of posttensioning where one end, 6 to 8 feet long, must be imbedded in concrete which when hardened, will mechanically interlock with the bare, clean strand to the extent that the other end can be hydraulically jacked to exert a 30,000 pound pull on a 1/2" tendon. One application for this type of tendon is the concrete transmission poles used in long distance power transmission. A typical pole may require 32 tendons in the 30" square base and only 12 in the 10" square top. Twenty tendons are imbedded in a staggered pattern in that section that is 30 to 60 feet from the base. In spite of all efforts of wiping, brushing, dissolving with solvents or heating, the grease applied to present tendons remains on the strand to such extent that 8 feet exposed to the concrete will not be gripped sufficiently to prevent pulling the strand out of the concrete during post-tensioning and the transmission pole is a total loss.
Also because most tendons are intentionally placed in an arc or curve, the strand during initial stressing forces the grease away from the point of contact of strand to plastic jacket and as tensioning continues, the strand, which is elongated about 8.4 inches per 100 feet of length rubs and abrades through the comparatively fragile plastic jacket to the concrete which adds greatly to friction and results in damage to the strand. In addition, the use of grease or corrosion inhibitor having grease-like consistency is widely used for its anti friction property but provides no support to maintain centrality of strand in tendon during tensioning.
It is therefore an object of the present invention to provide a tendon of improved construction which overcomes the above-discussed prior art shortcomings. This and other objects are accomplished by the provision of a tendon for use in poststressing concrete comprising a multiple-wire strand wherein individual peripheral abutting wires define a roughly geometric overall cross-sectional strand configuration having both internal and external interstices wherein said interstices are open ended crevices defined between adjacent wires, an encasement formed of a dielectric plastic completely surrounding the outside of said strand to form a smooth outer surface of circular cross-sectional configuration and having internal portions extending into said strand crevices; a thin friction reducing layer of grease-like material on the outer surface of said encasement and a loose plastic jacket disposed over said encased strand whereby said strand may freely move longitudinally relative to said jacket.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawing.